Monitoring of Neuromuscular Block Flashcards
Does TOF stimulation require a control twitch?
No
What kind of block is fade a feature of?
Non-depolarizing only
What should the TOF count be greater than to ensure reversibility?
3
How frequently can you perform a TOF stimulation?
It shouldn’t be performed any more frequently than once every 10 seconds, otherwise it can lead to recovery from block in the stimulated muscle
What kind of neuromuscular monitoring is accelomyography best suited to?
TOF and PTC monitoring
What must the TOF ratio be fore acceptable recovery?
>0.9
Which is more sensitive, DBS or TOF?
DBS is easier to assess clinically than TOF so is more sensitive
Which muscle is more accurate in reflecting block recovery, the adductor pollicus or the orbicularis oculi?
The adductor pollicis
Which recovers from block faster, the diaphragm or the adductor pollicis?
The diaphragm
Can you monitor DBS with acceloromyography?
DBS cannot be reliably monitored using acceleromyography
What is a potential source of error with EMG recording?
EMG recording may detect direct muscle stimulation.
What is the amplitude of an evoked EMG potential proportional to?
The amplitude of an evoked EMG potential is proportional to the number of neuromuscular junctions stimulated.
What preload is advised to be placed on the adductor pollicis muscle when using MMG monitoring?
200-300 grams
What is the resistance of skin in ohms?
0 - 5 kOhms
What disease states produce increased skin resistance?
Diabetes mellitus Hypothyroidism (thick skin)
How does temperature affect skin resistance?
Cold increases resistance Heat decreases resistance
Will skin resistance be affected by the type of electrode used?
Yes, the surface area and quality of contact varies with different electrodes
What does DBS consist of?
2 bursts of tetanus at 50Hz Each impulse separated by 20ms and the 2 bursts separated by 750ms
What is profound block best assessed by?
PTC
What issues can residual neuromuscular block cause?
- impairs ventilatory response to hypoxia
- increased risk of aspiration
- airway obstruction
What criteria must a nerve have to be used to assess degree of neuromuscular block?
- must have a motor element
- must be close to the skin
- contraction in the muscle/muscle group which the nerve supplies must be visible or accessible to evoked response monitoring
What current needs to be applied to generate a response through all the nerve fibres and hence make a maximal muscle contraction?
- current for 0.1 - 0.3 ms
- usually 25% above the maximal stimulus (supramaximal)
What properties would the ideal nerve stimulator have?
- battery operated
- able to deliver a constant current up to 80 mA
- adequacy of the electrical contact should be displayed on the monitor screen
- pulse stimulus should last no more than 0.3 ms and be monophasic, square wave type to ensure constant current throughout the stimulus
- polarity of electrode leads should be indicated
- should be able to deliver a variety of patterns - single twitch 1Hz, TOF 2Hz with 10s between trains, tetanic at 50Hz for up to 5s and DBS
- be able to monitor evoked responses
How does single twitch stimulation work?
- needs a control twitch before giving neuromuscular blocker
- single square wave supramaximal stimulus is applied to a peripheral nerve for 0.2 ms at regular intervals and the evoked response is observed
- twitch will be depressed when neuromuscular blocking agent occupies >75% of the post-synaptic nicotinic receptors
- twitch depression must be >90% to provide good conditions for abdo surgery
- useful at the onset of neuromuscular block
- can also be used in the post-tetanic count but don’t need a control twitch height for that
How does TOF stimulation work?
- freq 2Hz (4 stimuli separated by 0.5 s)
- then repeated every 10s (train freq 0.1 Hz)
- can compare T1 to T4 (TOF ratio)
- non-depolarizing agents produce fade, T4 reduces in amplitude, followed by T3, 2 and 1
At what % of depression of T1 does T4 disappear at?
T4 disappears at 75% depression of T1
At what % depression of T1 do T3 and T2 disappear at?
T3 disappears at 80-85% depression of T1
T2 disappears at 90% depression of T1
How many twitches should be present for reversal of residual neuromuscular block?
TOF count of 3 or more
What TOF ratio should be achieved before tracheal extubation?
0.9
What is phase 2 block?
If you give repeated doses or an infusion of succinylcholine then you will get features of a non-depolarising block such as fade
What is tetanic stimulation?
- uses high freq (50-200 Hz) with a supramaximal stimulus for a set time (5 seconds)
- the response is a tetanic contraction
- after giving non-depolarizing NMB sustained muscular contraction won’t be possible due to fade - which corresponds with degree of NM block
- very sensitive
- can elicit minor degrees of NM block
- limited use as extremely painful
What is the response to tetanic stimulation in a partial depolarizing block?
Fade is not observed.
The amplitude of evoked response will be lower but tetanic contraction is maintained
What is a post tetanic count?
- during profound NDNMB there may be no response to TOF or single twitch stimulation
- if a 5s 50Hz tetanic stimulus is administered, after no twitch response has been elicited, followed 3 s later by a further single twitch at 1 Hz there may be a response
- this will be seen before TOF reappears
What is post-tetanic facilitation?
- on completion of tetanic stimulus, ACh synthesis and mobilization continues for a short time
- as a result there is an increased store of ACh which causes an enhanced response to subsequent single twitch stimulation
- the number of post-tetanic twitches is an indication of when the first twitch of the TOF will reappear
- the first twitch will normally return with a PTC of 9 when using atracurium or vecuronium
When is PTC useful?
When profound neuromuscular block is required, for example during retinal surgery, when movement or coughing could have devastating effects
How long should you wait between tetanic stimulation?
6 minutes minimum.
Because if 2 PTCs are administered in quick succession the degree of NMB will be underestimated.
What is DBS?
- developed to detect even small degrees of NMB
- fade with DBS is easier to appreciate clinically than with TOF
- 2 short bursts of tetanus at 50Hz at supramaximal current
- each burst lasts 0.2 ms, separated by 20 ms with two bursts separated by 750ms
- if NDNMB is used - the response to second burst is reduced
- the ratio is the DBS ratio
What is mechanomyography (MMG)?
- measures evoked muscle tension
- most commonly studied is adductor pollicis in the thumb
- if the thumb is stabilized and placed under a fixed amount of tension (preload) then evoked responses can be measured as a change in tension develops
- uses a strain gauge transducer and recorder
- thumb doesn’t move - contraction is isometric
- this is the gold standard for assessing any nerve stimulation
- disadv - cumbersome, impractical for theatre
What is electromyography (EMG)?
- adductor pollicis or ulnar nerve most commonly used
- records a compound AP that occurs during muscular contraction
- evoked APs are a measurement of electrical changes that occur in muscle during stimulation - these are the equivalent to the muscular contraction that occurs after excitation-contraction coupling
- requires 3 electrodes
- creates a number of low voltage motor APs - summated into a compound AP which must be amplified
What are the disadvantages of EMG?
- prone to interference esp from diathermy
- hand temp/movement will interfere
- prone to drift
- another potential source of inaccuracy is direct muscle stimulation
What is acceleromyography?
- similar principle to MMG, but instead of measuring force of contraction directly, acceleration of the contracting muscle is measured
- force calculated using Newton’s second law of motion ( force = mass x acceleration)
- acceleration is measured using a piezoelectric ceramic wafer strapped to the thumb - produces a voltage proportional to acceleration when moves
- then converted to electrical signal and displayed as twitch response
- PTC can also be measured this way, but not DBS or tetanus
Which muscles is onset and offset of a block faster in?
Central muscles with good blood supply
eg diaphragm and larynx
Conversely those with poor blood supply (peripheral muscles) eg adductor pollicis
How do the muscles of the upper airway and pharynx behave at the onset of neuromuscular block?
At onset they behave as central muscles so the block works faster, but are sensitive to NMBs and recovery is slow, mirroring peripheral muscles
Why is the orbicularis oculi the ideal muscle to monitor at induction of anaesthesia and tracheal intubation?
- it’s similar to a central muscle
- onset of block will be similar to laryngeal muscles and diaphragm
- single twitch/TOF is most valuble at induction
- disappearance of TOF corresponds to optimal intubating conditions
What conditions are neuromuscular monitoring essential in?
- after prolonged infusions of NMB drugs or when long-acting drugs are used
- when surgery/anaesthesia is prolonged
- when inadequate reversal may have devastating effects, eg severe resp disease/morbid obesity
- when giving reversal may cause harm (eg tachyarrhythmias/cardiac failure)
- liver/renal dysfunction where pharmacokinetics of muscle relaxants may be altered
- neuromuscular disorders such as myaesthenia gravis or Eaton-Lambert syndrome
What preload should be used in MMG monitoring?
200-300g
What is the amplitude of the evoked EMG potential proportional to?
To the number of neuromuscular junctions stimulated
What does skin resistance vary from?
0 - 5 kOhms
What is profound block best detected by?
PTC is most accurate for this
